The demand for services offered over satellite communication systems is on the rise. For example, the ubiquitous coverage offered by satellite systems is highly desirable for various communication applications such as personal communication services, Internet access, local loop services, long distance services, cable television, commercial television, military operations and many others. In response to this demand, a number of large-scale satellite systems are scheduled for deployment in the near future.
As with all wireless communication systems, spectrum bandwidth is a valuable and limited resource in satellite systems. The term "spectrum bandwidth" refers to the difference between the two limiting frequencies of a band expressed in Hertz (Hz) For example, if a satellite system was assigned a range of frequencies between 2 GigaHertz (GHz) and 3 GHz, the bandwidth of the system would be considered 1 Ghz. Consequently, satellite systems attempt to utilize the bandwidth allocated to a system as efficiently as possible to improve the capacity of each system. The term "capacity" loosely refers to the system's overall potential for carrying or communicating information at any one point in time. This is sometimes measured by the number of users a system can service, or the total amount of information a system can transmit for all users simultaneously.
Satellite systems utilize a variety of techniques to improve bandwidth efficiency and system capacity. For example, satellite systems use multiple spot beams across the coverage area to facilitate spectrum reuse. Within each spot beam, satellite systems employ a number of different medium access techniques such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), Space Division Multiple Access (SDMA), and so forth, to further share existing bandwidth with multiple users. Bandwidth is shared by dividing the total system bandwidth into logical groupings referred to as "information channels." The definition of a channel varies with the medium access technique used. For example, a satellite system using FDMA divides the total system bandwidth into subsets of frequencies, with each subset being considered an information channel. If the satellite system uses TDMA, then a channel would be a specific time slot. If the satellite system uses CDMA, then a channel would be a power level assignment labeled with a spreading code.
Satellite systems use information channels to carry information signals from one user to another user engaged in an interactive two-way communication, such as in a telephone conversation or facsimile transmission, or from one user to multiple users, such as in teleconferencing. A communications connection between users is referred to as a "call connection." These information signals are typically generated during a discrete time period, which is usually measured from the time a connection between users is initiated to when the connection is terminated ("call session"). It is normal to have satellite systems carrying information signals generated during call sessions from multiple call connections, simultaneously.
For a majority of satellite systems, the information channels can only transmit information in one direction ("one-way channel"). This is due primarily to current satellite network architecture and the type of equipment used within this network and terrestrial telephone networks. For example, most communication networks use digital repeaters and electronics that only provide transmission in one direction, such as broadcast satellite networks used for TV signal broadcasting. Broadcast satellite networks typically use one-way channels to distribute TV programs from a source location to multiple users.
One problem with conventional satellite systems employing one-way channels is that they are designed to use two one-way channels to provide full duplex interactive two-way communications between a pair of users, such as when completing a telephone call. This results in an inefficient use of bandwidth, as illustrated in more detail with reference to FIG. 1.
FIG. 1 illustrates a conventional satellite system, comprised of two communication sites. Each communication site includes a terminal 200, an earth station 202, and an antenna 204. Each communication site is connected via a satellite 206 using two one-way channels numbered 208 and 210, respectively.
As shown in FIG. 1, when user A calls user B using terminal 200, the two one-way channels 208 and 210 are established for the call. Channel 208 will be referred to as channel 1, and channel 210 will be referred to as channel 2. Channels 1 and 2 each have an up-link channel segment and a corresponding down-link segment to the receiving party. When user A speaks, the voice signals are transmitted using channel 1 on the up-link band (1U) to satellite 206, and continue on a corresponding channel 1 to user B in the down-link band (1D). Similarly, when user B speaks, the voice signals are transmitted on channel 2 in the up-link band (2U) to satellite 206, and down to user A on channel 2 in the down-link band (2D).
As shown in FIG. 1, conventional satellite systems must utilize two one-way channels to establish interactive two-way communications between users, with one channel being used for transmitting signals in one direction (e.g., from A to B) and another channel being used for transmitting signals in the opposite direction (e.g., from B to A). The inventors of the present invention, however, have recognized that in a typical voice conversation between users, one user remains silent while the other user speaks, and vice-versa. These periods of silence on average account for over 50% of the time during a typical call session. As a result, each channel in effect is used only one-half the time during a call session. The use of two one-way channels for a single call session, therefore, leaves one-half of the overall capacity of the system unused during normal two-party calls.
In view of the foregoing, it can be appreciated that a substantial need exists for a method and apparatus which improves the capacity of satellite systems by making use of the unused capacity created during a two-way interactive call connection using two one-way communication channels.